1. Field of the Invention
The present invention relates to an LCD (Liquid Crystal Display) device using an AFLC (AntiFerroelectric Liquid Crystal), and more particularly to an AFLC display device which is capable of performing gradation display and a method for driving the display device.
2. Description of the Related Art
A ferroelectric liquid crystal display device using an FLC (Ferroelectric Liquid Crystal) has received attention due to its quicker response and wider angle of visibility than those of a TN liquid crystal display device using a nematic liquid crystal.
Known FLC display devices include an FLC display device which uses an FLC and an AFLC display device which uses an AFLC.
An AFLC display device displays images through utilization of three stable alignment states which an AFLC has.
This matter will now be discussed in more detail. An AFLC has the first to third stable states in regard to the alignment of LC (Liquid Crystal) molecules; (1) in the first and second stable states, when a voltage equal to or higher than a first threshold value is applied to an AFLC, the liquid crystal exhibits a first ferroelectric phase wherein the LC molecules are aligned in a first direction or a second ferroelectric phase wherein the LC molecules are aligned in a second direction, in accordance with the polarity of the applied voltage; and (2) in the third stable alignment state, when a voltage equal to or lower than a second threshold value less than the first threshold value is applied to the AFLC, the liquid crystal exhibits an antiferroelectric phase which differs in the alignment of the liquid crystal molecules from the first and second ferroelectric phases. Determining the directions of the transmission axes of a pair of polarization plates arranged one on either side of an LCD device, on the basis of the optical axis of a liquid crystal layer while in the antiferroelectric phase, enables a display device to display images, with controlling a light transmittance in accordance with the applied voltage.
Even when a variation occurs in the applied voltage, an AFLC stays in the first/second ferroelectric phase or the antiferroelectric phase, as far as the value of the applied voltage lies in a range between the first and second threshold values. This property is known as a memory property. A conventional AFLC display device is driven in a simple matrix mode through utilization of this memory property.
The memory property of an AFLC is determined by a difference in magnitude between a voltage, which causes a phase transition from the first/second ferroelectric phase to the antiferroelectric phase in the liquid crystal, and a voltage which causes a phase transition from the antiferroelectric phase to the first/second ferroelectric phase in the liquid crystal. The greater difference between those voltages, the more excellent memory property. In other words, the more remarkable hysteresis of the optical characteristic of the liquid crystal, the more excellent memory property.
Due to this, a conventional AFLC display device which is driven in a simple matrix mode uses, as an AFLC, such a liquid crystal that the difference between the magnitudes of the above-described voltages is large.
However, the light transmittance of the conventional AFLC display device, which uses an AFLC having an excellent memory property, can hardly be controlled arbitrarily. That is, the control of a display gradation is almost impossible and multi-gradation display cannot be realized.